SLP Headers

I put SLP long tube, tuned length headers on my 99 Camaro SS and things were fine for almost a year. Then I got the "service engine soon" light on occaisionally for while. It worked it's way up to more frequent and then full time light "on". Checked codes and found "cat II decreased efficiency" code. Any idea if this is the right or left cat? Cat's cost almost 400 and came from SLP also, would hate to have to replace them. In the meantime, I bought O2 simulators and hooked them up and that got rid of the engine light. (if you hook them up wrong i.e. to the fwd O2 sensors your car will run very poorly and numerous messages like misfire and MAP sensor will come up on computer)

Hook the O2 sims up to each of the rear O2's.
Or delete them from the computer all together like I did.

Hi-flow cats are great, but they don't do enough for the computer. They will throw a code.
I have the off-road headers, hi-flow cats, and y-pipe from SLP on my car. Well did. I had the O2's deleted from the computer not 3 days after the header install. I hate that light.
Personally, I am ditching the SLP's for Kooks. Not for performance, but SLP doesn't make a 1 7/8" primary header. Plus, a stainless header rules.

to answer the question, everything number one is driver side, and everything 2 is passenger side. So your passenger side cat is going bad.